The present invention relates to a heterostructure optical operator having quantum wells, particularly, but not necessarily, an operator such as a photorefractive operator.
In a photorefractive (PR) crystal, two incident coherent laser beams produce an interference pattern which writes an index grating. The grating can then be used in turn to deflect a third incident beam in a direction that depends entirely on the properties of the grating. The suitability of PR materials for having an index grating written optically therein, and thus enabling the deflection of a laser beam to be controlled, makes such materials particularly advantageous for switching optical signals in a broadband telecommunications network. For example, a matrix having N fiber inputs can be interconnected with a matrix having M fiber outputs. Another particularly advantageous technological application lies in parallel and ultrafast processing of images for pattern recognition and for neural networks.
Most studies of these applications concern solid PR crystals. In this context, reference may be made to the reference work:
[1] "Photorefractive materials and their applications I", by P. Gunter, J. P. Huignard, Springer, Berlin, 1988.
Nevertheless, solid crystals suffer from two major drawbacks, namely relatively long response times (of the order of a millisecond), and very strict alignment requirements, due to the phenomenon of Bragg diffraction.
These problems have led several teams throughout the world to propose thin-layer photodiffractive devices based on quantum wells. In devices of that type, the incident interference pattern induces considerable modulation of the refractive index and of the absorption coefficient (respectively .DELTA..eta. and .DELTA..alpha.) by means of high electro-optical coefficients about the forbidden band of the quantum wells. Such devices naturally have very short response times (of the order of one microsecond), and their short optical length makes it possible to avoid the Bragg diffraction phenomenon.
With respect to such thin-film photorefractive devices known in the prior art, reference may advantageously be made to the various following publications:
[2] Q. Wang, R. M. Brubaker, D. D. Nolte, and M. R. Melloch, J. Opt. Soc. Am. B9, 1626 (1992) together with the references cited therein;
[3] A. Partovi, A. M. Glass, D. H. Olson, G. J. Zydjik, H. M. O'Bryan, T. H. Chiu, and W. H. Knox, Appl. Phys. Lett. 62, 464 (1993); ibid 59, 1832 (1991);
[4] G. Picoli, P. Gravey, and J. E. Viallet, French patent application published under the number 2 678 093.
The photodiffractive devices that have been described in the past assume that an external electric field is applied parallel to or perpendicular to the plane of the quantum wells. The perpendicular configuration is the most advantageous because of the significant electro-optical effect obtained by the quantum confined Stark effect (QCSE). In this respect, reference may advantageously be made to the following publication:
[5] D. A. B. Miller, D. S. Chemla, T. C. Damen, A. C. Gossard, W. Weigmann, T. H. Wood, C. A. Burrus, Phys. Rev., B32, 1043 (1985).
In the device described in [3], the photodiffraction mechanism may be analyzed as follows:
1) The wavelength is adjusted to a value close to exciton resonance with high levels of absorption.
2) The photo-created electron-hole pairs in the wells pass through the barriers and are attracted to the electrodes by the electric field.
3) The carriers close to the electrodes are trapped and screen the applied electric field, creating large .DELTA..eta. and .DELTA..alpha. by the quantum confined Stark effect.
4) If photoexcitation is created with an interference pattern, then a .DELTA..eta. and .DELTA..alpha. array is obtained in phase with the intensity pattern of the laser. In this operating plane, although the trapping phenomenon at the interface between the electrodes and the semiconductor is not manifest, these researchers have found relatively high diffraction efficiencies of 3%, but with the important restriction that the period of the grating must remain relatively large (30 .mu.m). When the grating period is reduced by a factor of 10, then the diffraction efficiency is rapidly reduced by a factor of 1000. This has to do with the lateral diffusion of the carriers that takes place, either during displacement towards the electrodes, or else after trapping at the electro-semiconductor interface (erasing the grating).
Various solutions have been proposed to obtain better microscopic understanding and better control over the trapping mechanism. For example, FR-A-2 678 093 envisages epitaxial growth of a semi-insulating InP:Fe layer having a thickness of a few microns on the layer having multiple quantum wells, where InP:Fe is known for its photorefractive properties and is used as a medium in which gratings can be written. Gratings can be written in this way either by direct photoexcitation of deep centers beneath the forbidden band of the multiple quantum wells, or else by photogeneration of carriers in the multiple quantum well region, where the carriers then escape and are trapped in the InP:Fe layer. In FR-A-2 678 093, the unit pattern is repeated to obtain a larger effect. If analysis is restricted to the operation of a single pattern only, then the diffraction efficiency is limited because of the value of the diffusion field in solid InP:Fe (about 1 kV/cm). Under such circumstances, that means it is necessary to use an external electric field to improve the diffraction efficiency.
One way or another, all of the above-specified studies rely on an external electric field being applied. That requires lengthy and difficult processing to be applied to the samples, which imposes further constraints on the performance of the device, on its suitability for being integrated in a system, and on the possibility of miniaturizing it.